Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Preparing to start polishing and this happens.... 
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Chris
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 13776
Loc: Amargosa Valley, NV, USA
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Oh, that hurts even to look at! 
Is it salvageable? The blank looks pretty thin. Can it be reground on the other side?
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"Since the process of science generates more mysteries than it solves, we can never learn everything, we'll simply generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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Don W
Demi-God
Reged: 05/19/03
Posts: 15145
Loc: Wisconsin, USA
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Eeeeek!! Doesn't look like the edge was beveled enough.
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Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
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Scott Horstman
Vendor- Backyard Observatories
Reged: 03/11/04
Posts: 8155
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Oh no!! That just stinks!!!
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Scott
www.backyardobservatories.com
www.m1oasys.com
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RRPG
sage
Reged: 01/26/08
Posts: 363
Loc: 42° 25'50" 123° 19'00"
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I am pushing a thin 13.5 blank and feel your pain.
“Only after disaster can we be ressurected.”
Chuck Palahniuk
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Peter
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8 in HO Truss Dob
8 in. Skywatcher SCT
C102XLT
ETX 90 EC
C80ED
CG5-GT
WIP: 14 inch HO Truss Dob
Canon 450D Xsi
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GJJim
scholastic sledgehammer
Reged: 09/09/06
Posts: 907
Loc: Western CO
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That is too bad, especially for it to happen after all that fine grinding. That chip is huge -- did you bump the edge while cleaning?
At this point starting over on the reverse side is probably your best bet.
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gnabgib
sage
Reged: 06/05/05
Posts: 371
Loc: Fall River MIlls Ca.
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Just bevel the edge more (at least 1/8 inch and rember a rounded edge rather then a 45 degree will cause any chip to rise toward the surface quicker resulting in a smaller chip).
The loss is only a few square inches and a HUGE blow to ones pride. When observing you will never know it is there.
Keep on polishing!!!!!
My .02 worth
Kevin
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 920
Loc: Easton, PA, USA
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Chris,
I'm very sorry to see that.
What will be done?
I hope the maker isn't discouraged from continuing.
Gary
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1671
Loc: Fort Worth TX
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Chris,
Kevin's absolutely right. The effect on the image will be minimal, mostly just a few percent in light loss. That chip's not even the size of the diagonal you'll use. Doesn't look like you have enough blank thickness to grind the other side without getting too thin at the center, and it's not worth doing anyway. The only problem is having to trim out the lap so it doesn't flow into the chip during pressing.
Once it's figured and aluminized, a little ultra-flat black and a Q-tip will eliminate any trace of the chip.
Tell you what - take a photo of it at normal incidence and post it, and I'll curve-fit it and run a ZEMAX simulation of the chip and resulting star images so you can see for yourself. Remember Frank Tomazewski's 10" mirror with all the chips around the edge, and the analysis I did for it last year? If you added up the surface area of all of Frank's chips, they probably were about the same total area as this one. The effect on the PSF was insignificant.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Thanks Guys,
... for the supporting words, 'lucky' me...  .. the mirror is not mine. This mirror is Jean-Pierre's. An ATM-friend here in Belgium from the Oostende region. Not to far from our local observatory. He also gives a telescopemaking course in that area.
He will continue polishing and using it. At first he wanted to cut it down to 18" to get rid of the nasty chip. His pride got a bit of a blow for a few hours but I adviced him to continue. And he will. He will cover it up with some black paint once the mirror is done.
The mirror is indeed to thin to start all over at the reverse side, it's only 25 mm thick, but it would not have been an option anyway.
For Jean-Pierre there is no way he will be Flexing his mirror. he's forced to parabolize it the old fashion way.. pitty.
Thanks Mike for his kind offer, I'll ask JP to make a new picture and will post it here
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Chris
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3401
Loc: Northeast
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"For Jean-Pierre there is no way he will be Flexing his mirror. he's forced to parabolize it the old fashion way.. pitty."
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What? Think of it as a learning experience!
M.
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So many projects, so little time!
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F.Meiresonne
Post Laureate
Reged: 12/22/03
Posts: 3632
Loc: Eeklo,Belgium
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What a pity Chris, it's horrible to look at it. But disasters do happen. Here in Assenede a 18" broke in 2 pieces once while grinding...ouch...
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Freddy Meiresonne
Obsession 18 inch #1638
Orion Optics 8 inch F/4.5 -1/8 wave optics -Vixen GP-E
22x85 Helios Apollo (=GO SS)
15x70 TS Marine (=Obie Ultra)
10x60 Helios Quantum 4(= Obie Mariner)
10x50,8x40 Helios Nature sport plus
Eyepieces in use :Pan 35,24,19, N13T6, Pentax 10 XW, N9T6, Ultrascopic 7.5, TV2, BGO 12.5 and 9 mm
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Quote:
"For Jean-Pierre there is no way he will be Flexing his mirror. he's forced to parabolize it the old fashion way.. pitty."
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What? Think of it as a learning experience!
M.
Mark,
JP knows how to parabolize very well, he was going to try his very first Flexed mirror... 
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Chris
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Quote:
What a pity Chris, it's horrible to look at it. But disasters do happen. Here in Assenede a 18" broke in 2 pieces once while grinding...ouch...
I know Freddy, I rememeber it was posted here at CN a while ago.
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Chris
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oldphoggie
newbie
Reged: 01/09/09
Posts: 4
Loc: Ohio
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What were the circumstances that caused it? A bigger bevel would have helped? You have my sympathy.
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8681
Loc: Bremerton Washington
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Quote:
Quote:
What a pity Chris, it's horrible to look at it. But disasters do happen. Here in Assenede a 18" broke in 2 pieces once while grinding...ouch...
I know Freddy, I rememeber it was posted here at CN a while ago.
Being familiar with mirror disasters...I feel for you guys.
To get so close to the final product and.........
Rob(been there, done that)
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Mike,
This the picture JP send me. I hope it is large enough to be used in ZEMAX. If not let me know I ask for a larger one.
Thanks....
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Chris
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3401
Loc: Northeast
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I'll almost bet that it was a wicked clunk up against something hard, or it might have hit the floor.
M.
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So many projects, so little time!
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Quote:
Mike,
This the picture JP send me. I hope it is large enough to be used in ZEMAX. If not let me know I ask for a larger one.
Thanks....
Silly me ... 
Forgot the picture.
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Chris
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kfrederick
scholastic sledgehammer
Reged: 02/01/08
Posts: 783
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hi Chris in 1992 i had a 20 inch pressing on a lap[ mirror on top ]with 50lbs of weights on top /it slid off and landed on the floor [carpet over wood floor] i was in another room when it fell / no damage //i was very luckey / i have neaver pressed with the mirror on top sence/ kevin
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Quote:
I'll almost bet that it was a wicked clunk up against something hard, or it might have hit the floor.
M.
Not at all Mark,
OK I will reveal the 'secret'....
JP was told to glue a second glasdisk at the bottom of its mirror before polishing. The plan was to glue that disk with beeswax. JP applied a coat of beeswax on top of the second glassdisk put his mirror facing up on top of that second disk and started to heat the 20" 'sandwich' in a belljar. It was done in a belljar to avoid uneven warming up of the disks (If I'm not mistaken). Problem was the disk fitted in the belljar with almost NO play. Once the beeswax was set the glassdisk were removed.
But disaster struck. While removing the pair of glassdisks both disks got stuck in the belljar. Trying to unjam the mirror out of it's cramped situation the chip broke loose.
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Chris
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Olivier Biot
Amused
Reged: 04/25/05
Posts: 17330
Loc: 51°N (Belgium)
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What's worse is that they could probably have released the disks from the bell jar if they waited for sufficient time.
In the case the wax effecively sealed the bell jar, I would have tried breaking the vacuum with a very tiny needle, and maybe by applying heat only around the bell jar by means of a dew heating strip. This way the wax would liquefy again and the bell jar would slightly expand while the glass disks would not significantly expand.
Just my 2 cents.
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Computer entomology: study of computer bugs
Tal-200K (#199) with JMI NGF-Mini2M focuser on GEM3 • Astro-Tech AT80ED • Orion Sirius EQ-G with wireless EQDIRECT • Astro-Tech Voyager • Celestron Regal LX 10x42 • Helios 15x70
ATM 14" f/5 (redesigning) • ATM 10" f/6 Portable Truss (polishing) • ATM 10" f/25 Dall-Kirkham (optics)
AstroForecast
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1671
Loc: Fort Worth TX
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Chris and Jean-Pierre,
Here's the Excel digitization of the boundaries for the mirror and chip.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1671
Loc: Fort Worth TX
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Here are the aperture functions and resulting PSF (star image) log plots (which show more detail than the eye can see) for a 20" f/5 mirror at 0.55um. I assumed a 5" minor axis diagonal and 4-vane 0.1" wide struts (a little on the thick side). The effect of the chip is far less than the inclusion of the CO and spider. The effect of the chip is almost vanishing on star imagery.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1671
Loc: Fort Worth TX
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Reducing the gamma of the above picture more realistically simulates the visual appearance of the PSF. I turned it down so that one Airy ring is visible but the outer rings barely show up.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1671
Loc: Fort Worth TX
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Finally, here are the MTF curves for the 20" f/5 clear aperture, the aperture with the CO + Spider, and the CO + Spider + Chip. Easy to see that the addition of the CO and spider (and mostly the CO) contribute the majority of the contrast reduction below the MTF of the clear aperture (as good as it gets). The chip MTF curve in green almost over-writes the CO+Spider curve all the way to cutoff.
I calculated the area of the chip from the Excel digitization to be about 5.26 in^2. The total area of the mirror is 314 in^2, so the chip only reduces the light collecting area of the mirror by about 1.68%.
As long as chips don't screw up the mirror figure (potato chipping around the edge from released strain), and they are painted flat black to minimize forward scattering, their effect on imagery is almost zero, at least for most telescope applications. Instrumentation or observing programs requiring maximum contrast, such as extrasolar planet imaging, spectrometers or coronascopes, would have to have very clean, chip-free objectives.
Mike
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PeterKA
journeyman
Reged: 05/12/06
Posts: 7
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I was polishing an 8 inch blank (a decade ago) and in moving it to under a water faucet for rinsing, the edge just glanced off the spigot - neatly removing a divot similar to this one.
I agree that in a finished mirror, the effect would not be disastrous.
But I don't know how a mirror like this can be polished. Yes, you can cut away the lap, but your strokes would have to be very short and not randomized. And the area around the chip would not be polished or parabolized. Obviously I'm missing something.
thanks
Peter Abrahams
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Thanks Mike,
Really impressive to see this diagram.
As I see it in the graph no reason to worry about...
In a way I think JP will be pleased to seeing this result.
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Chris
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Yep,
He's very pleased with your posts Mike, he asked me to thank you for it... 
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Chris
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sixela
Postmaster
Reged: 12/23/04
Posts: 11499
Loc: Boechout, Belgium
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Quote:
Reducing the gamma of the above picture more realistically simulates the visual appearance of the PSF. I turned it down so that one Airy ring is visible but the outer rings barely show up.
To make the PSF well nigh identical (less "wiggly"), you could of course make the blackened surface closer to half-circular. Not that it matters that much, but the first Airy ring would probably be more point-symmetrical (and *really* hard to discern from the original).
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3401
Loc: Northeast
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Wouldn't the presense of the chip affect the flexed mirror's symmetry after installed in a cell? It certainly can't be so "cut&dried" this easily.
M.
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So many projects, so little time!
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sixela
Postmaster
Reged: 12/23/04
Posts: 11499
Loc: Boechout, Belgium
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I doubt it'll matter that much. Obviously, you don't want an edge support point near that chip, but other than that I don't think it's going to matter. Provided the figure hasn't changed and that no stresses have built up in the rest of the glass (but chips are usually the way those stresses were relaxed).
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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CoreDump
member
Reged: 11/06/06
Posts: 55
Loc: France
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Hi,
I will ask myself a couple of questions before making a decision:
Is the blank value worth the risk of being bad once polished?
Is the blank value higher than the difference in resale value between a perfect 20" and a chipped one?
Can I trim the blank and resale it for financing a new one?
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Adventurer
professor emeritus
Reged: 11/22/07
Posts: 731
Loc: Amman, Jordan
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A bet too late here, I am so sorry for that chip, but I am sure you will end up with nice mirror that will substitute this fracture. 
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"man is not made for defeat....A man can be destroyed but not defeated"
Regards
Rasim Ramadan
Home made 10 inches reflector.
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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It'll be not doable to flex that mirror. The hole consept is based on perfect symmetry. If one or other component has a minor flaw it'll not work at all.
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Chris
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3401
Loc: Northeast
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That's what I thought. Too bad, you guys like those flex mirrors.
Mark
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So many projects, so little time!
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kfrederick
scholastic sledgehammer
Reged: 02/01/08
Posts: 783
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CHRIS thanks for all the good info , so you think that chip will change the glass enough to making flexing not work ? be cool to see what it does/ i neaver tried flexing/ how big do you think it can be done ? great info kevin
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Owen
professor emeritus
Reged: 06/21/07
Posts: 519
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Could you not mask and flex - using a sub diameter ring..?
Just a thought
Owen
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Quote:
CHRIS thanks for all the good info , so you think that chip will change the glass enough to making flexing not work ? be cool to see what it does/ i neaver tried flexing/ how big do you think it can be done ? great info kevin
No not at all Kevin,
The chip itself will not change the mirrors shape. It is the result of the pulling force in combination with the chip that will disform the mirrors surface during pullingaction.
How big...? Well, my ultimate goal is to grind a 40" mirror. Once I've done that I'll let you know whether a mirror that size can be flexed or not. But for that info you'll have to wait some time, because I plan to make some more smaller scopes. The biggest we've flexed so far is 20" , works great.
During pulling the mirror 'disforms' from sphere to ellips and if you keep pulling deformation to a severe hyperbola is perfectly doable.
Nice technique, pitty so few amateurs use it.
This very moment we're busy making a few DK's. The primary mirrors of these scopes will also be flexed. Busy making all the mecanical parts this very moment, almost done. It's not easy to flex a cored mirror. Will it work...? Don't know, I'll let you know in a few weeks...
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Chris
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